The present invention relates to a method for producing L-tryptophan by fermentation.
L-tryptophan, which is one of the essential amino acids and an essential component in human and animal nutrition, is known to be synthesized by a variety of routes such as starting with .beta.-indolylaldehyde and hippuric acid (Ber. 39, 2515, 1906) or starting with .alpha.-ketoglutaric acid and phenyl hydrazone (U.S. Pat. No. 3,019,232). Other than these chemical synthetic process, it is known that L-tryptophan is produced from its precusors such as an antranilic acid, indole or 3-indole pyruvate by an action of microorganisms.
It is also known that L-tryptophan is produced by a fermentation process in which L-tryptophan producing mutants of the genus Brevibacterium are used. And various L-tryptophan producing mutants, produced by the artificial mutation of wild strains of microorganisms of the genera Brevibacterium or Corynebacterium, are known. Examples of such artificial mutants are mutants resistant to tryptophan analogue such as 5-methyl tryptophan (Japanese Published Examined patent application No. 18828/1973), mutants resistant to tryptophan analogue and phenylalanine analogue, and mutants resistent to these analogue and further requiring L-amino acid such as L-tyrosine, L-phenylalanine, L-methionine L-histidine for their growth (Japanese Published Unexamined patent application Nos. 42091/1975, 129791/1975, Japanese Published Examined patent application No. 19037/1976 and Agr, Biol, Chem., 39, 343(1975)).
Recently, there is a great demand for L-tryptophan as a feed stuff but the demand can not be supplied because L-tryptophan can not be produced at a reasonable price by any fermentation process or chemical synthetic process even though various process for producing L-tryptophan are known as stated above.